Integrated Pest & Crop Management Newsletter University of Missouri-Columbia Vol. 15, No. 1 January 14, 2005 ************************** Glyphosate-resistant weeds By Kevin Bradley *************************** The following article was written concerning glyphosate-resistant weed development and was prompted, in part, by the of official announcement from Dr. Reid Smeda about the discovery of a glyphosate-resistant common ragweed biotype in Missouri. So far, this common ragweed population is confined to approximately 20 acres in a field with a history of glyphosate use in long-term soybean production. In initial greenhouse experiments, this biotype appears approximately 10 times more resistant to glyphosate than a susceptible common ragweed biotype. In field trials, the addition of lactofen (Cobra or Phoenix) to a standard in-crop glyphosate treatment significantly enhanced control of this species. It was also discovered that glyphosate-treated ragweed plants were infested with a stem boring insect called the ragweed borer, and it is not known if the insect compromised some of the glyphosate activity in the field allowing plants to survive. However, all greenhouse dose-response experiments were conducted in the absence of the ragweed borer. In addition, resistant common ragweed plants exhibit a shorter growth habit, and Dr. Smeda is currently conducting research to determine if herbicide retention is altered, possibly contributing to resistance. It is well known that glyphosate-resistant horseweed (also known as marestail) populations have been selected in Roundup Ready soybean and cotton cropping systems. Resistance was first reported in Delaware in 2000, a mere five years after the introduction of Roundup Ready soybean. Since that initial report, glyphosate-resistant horseweed is now reported in 12 states and is estimated to affect 1.5 million acres in Tennessee alone. A person could ask if this is any indication of what might lie ahead. On one hand, it has been proposed in a popular advertisement that glyphosate-resistant weeds are unlikely to occur if glyphosate is frequently used, as long as glyphosate is applied at full rates. The comments in this advertisement, in part, are based on several long-term university studies of Roundup Ready cropping systems. However, the question that a person should ask about these studies is whether they can prove that resistance will or won’t happen. It is our belief that these studies are not large enough to test whether resistance will develop. For example, the University of Wisconsin has a seven year Roundup Ready cropping system trial. This trial has horseweed in the no-till plots. Despite burndown and in-crop treatments with glyphosate, glyphosate-resistant horseweed has not developed in these plots. Since this trial did not find glyphosate-resistant horseweed, does this mean that glyphosate-resistant horseweed cannot develop? Does it mean that the resistant horseweed in Ohio or Tennessee or Delaware is not truly resistant? Obviously not. Small scale trials cannot prove that some event will not occur when a larger scale is considered. In total, these Roundup Ready cropping system trials may only be testing continuous glyphosate use on perhaps 50 acres, which is an extremely small fraction relative to total glyphosate. The true test to determine if a rare event like glyphosate-resistant weeds will develop is actually being tested on the tens of millions of acres of Roundup Ready corn, soybean, and cotton that are sprayed each year. Many weed scientists across the Midwest have warned of the potential for additional glyphosate-resistant weeds if a "high selection pressure" is maintained. In this case, high selection pressure refers to the repeated use of glyphosate without interruption by herbicides with other modes of action or other weed management practices. This potential was confirmed at the North Central Weed Science Society meeting where glyphosate-resistant common ragweed was officially reported. This is the first report of glyphosate-resistance for common ragweed. It has been identified in a Missouri no-till soybean field that has been in continuous soybean production (with some double crop wheat) for many years and in Roundup Ready soybean since 1996. Midwest weed scientists believe in the value that glyphosate and Roundup Ready crops offer to growers. We hope that growers and crop advisors will evaluate how they use glyphosate and the Roundup Ready technologies to gain the value of these technologies without increasing the risk of resistance. Ideally, we recommend: 1) Tank mixing glyphosate with another mode of action like 2,4-D in burndown treatments when glyphosate will be applied in the subsequent crop, 2) Alternating glyphosate use with other herbicide modes of action between years, and 3) Incorporating appropriate integrated weed management practices Kevin Bradley, Agronomy (573) 882-4039 ******************************* Soybean rust and crop insurance By Ray Massey ******************************* The USDA Risk Management Agency (RMA) that oversees crop insurance states that “losses to soybean production due to soybean rust disease is an insurable cause of loss provided the insured can verify that the cause was natural and available control measures were properly applied….The RMA will be…vigilant to monitor when outbreaks are detected in an area to determine if an insured could have applied recommended fungicides in a timely manner and did not.” It will be imperative for “producers to keep informed of soybean rust outbreaks in their area and take recommended measures to control or prevent the disease impact if an outbreak is anticipated or already in the area.” There are two major categories of crop insurance products for soybeans in Missouri. Yield insurance contains the familiar catastrophic plan, the actual production history plan and the group risk plan. All pay indemnities when yields are lower than a level chosen by the farmer purchasing the insurance. Revenue insurance contains crop revenue coverage and revenue assurance. These plans are more complex with indemnities being paid when either revenue or yields fall below a level chosen by the purchasing farmer. Seventy-five percent of soybean acres in Missouri were insured in 2003. For the last five years Missouri farmers have received more than $2 from insurance indemnities for every $1 paid in premiums (Note: 2004 crop insurance data are not yet published but I do not think it will pay as well given the yields and prices seen in 2004). With so many farmers purchasing and benefitting from crop insurance, considering the impact of Asian soybean rust on their crop insurance decision seems wise. Should the policy purchased this year be the same as last or does the added risk of soybean rust losses argue for a change? The answer lies, in part, on what you expect the impact of soybean rust to be in your area and in the nation. Historically the catastrophic plan pays poorly. Farmers who purchase this plan rarely receive an indemnity because the yield must fall below 50 percent of their 10 year average. If you believe, should soybean rust hit, it will devastate your crop, catastrophic insurance may be worthwhile. Be warned the RMA is expecting farmers to take recommended measures to protect their crop from soybean rust. If you believe that soybean rust, should it hit, will impact your entire county, Group Risk Plan (GRP) may be useful. GRP is not often purchased and pays only when the county yield is below a certain level. GRP is a good plan for those whose farm yields closely follow county yields. It is less expensive than other forms of insurance but rarely pays. Most farmers purchase revenue assurance (RA) rather than crop revenue coverage (CRC). CRC differs from RA in that CRC purchases benefit from higher harvest time prices should prices increase throughout the growing season. If you think that soybean rust will significantly reduce US soybean production increasing soybean prices, CRC would allow you to benefit from that price increase. As always, crop insurance decisions should be made in consultation with your financial advisor. The major impact of soybean rust may be that it causes you to rethink your crop insurance choices rather than just take the same level of coverage that you have had for the last several years. Any crop insurance decision for soybeans grown in 2005 must be made by March 15, 2005. Ray Massey, Ag Extension-Social Sciences (573) 884-7788 *************************************************************** Early-maturing soybean varieties often yield less than adapted varieties By Bill Wiebold *************************************************************** Because Asian soybean rust was found in the southern US in late 2004, Missouri farmers may be thinking about planting soybean varieties that mature earlier than varieties adapted to their region. Soybean is highly sensitive to photoperiod, so soybean varieties are adapted to narrow bands that run nearly east and west across the country. The soybean maturity groups associated with Missouri range from MG III in the north to MG V in the south. The reason that farmers may plant early-maturing varieties is that they hope that their soybean are plants further along in the seed-filling process before rust moves from the southern USA to their fields. This strategy makes sense because the effects of most stresses on soybean yield depend on the stage of development at which the stress occurs. Soybean plants affected by rust or other stresses in mid-seed filling (R6) suffer less yield loss than soybean plants affected by rust at beginning pod development (R4). In general, two varieties that differ by a whole maturity group will differ for the date on which they mature (R7) by about 10 days. Unfortunately, it is hard to predict variety response to in-season growing conditions and this difference might be fewer or more than 10 days. Stages earlier than R7 are of more interest to farmers, and we have little data upon which to base predictions. In central Missouri, we planted varieties ranging in maturity from 2.8 to 4.6 (mid-II to mid-IV). This experiment was conducted for only two years in one location so information is limited. We found that two varieties that differed by a whole maturity group differed for beginning flowering (R1) date by about five days and for beginning seed-filling (R5) date by about seven days. So, planting varieties from earlier maturity groups will affect dates at which plants reach important growth stages. However, yield potential of earlymaturing varieties is probably less than adapted varieties – at least in central and north Missouri. Again, Missouri data are very limited. The earliest variety used in the study discussed above, had special oil characteristics and its yield may not be indicative of other earlymaturing varieties. But, early-maturing varieties (e.g., MG I and II) planted in north or central Missouri will be shorter, branch less, and have smaller leaf areas than adapted varieties. For these reasons their yield potential will be predictably less. Farmers thinking about planting early-maturing varieties will find little data on which to base variety selection. They may use data from variety trials held in states to the north, but yield performance will likely change. Planting early-maturing varieties in central and north Missouri is not recommended. In SE and SW Missouri we have some data that indicate that MG III varieties may be an appropriate choice for a portion of the soybean acreage. However, yield variation from year to year may be greater for earlymaturing varieties than for adapted varieties, so predicting yield potential will be harder. Early-maturing plants will mature when weather conditions are hot and humid. Seed-borne diseases including phomopsis are likely to occur and grain quality may be reduced. Reduced grain quality may lead to lower grain prices. Bill Wiebold, Ag Extension-Plant Sciences (573) 882-0621 ************************************************* Weather Data for the Week Ending January 10, 2005 By Pat Guinan ************************************************* -------------------------------------------------------------------------------- | Monthly | Growing Weekly Temperature (deg. F) |Precip (in.)|Degree Days^ -----------------------------|------------|------------ Ext- Ext- Depart| Depart|Accum Depart Avg.Avg. reme reme from |Jan 1- from |since from Station County Max.Min. High Low Mean avg. |Jan 10 avg. |Apr 1 avg. ------------------------------------------------------|------------|------------ Corning Atchison 25 11 35 -1 19 -3 0.26 -0.11 * * St. Joseph Buchanan 27 15 40 3 22 -2 1.27 +1.02 * * Brunswick Chariton 31 22 39 10 27 +3 1.85 +1.42 * * Albany Gentry 26 15 37 4 22 0 0.79 +0.48 * * Auxvasse Audrain 34 25 50 15 29 +4 3.76 +3.28 * * Columbia Boone 35 25 51 14 30 +3 4.02 +3.58 * * Sanborn Field Boone 36 25 52 14 30 +3 4.24 +3.80 * * Novelty Knox 30 22 37 14 26 +2 2.04 +1.67 * * Linneus Linn 30 21 36 9 26 +3 1.55 +1.23 * * Monroe City Monroe 32 24 43 15 28 +2 2.96 +2.57 * * Versailles Morgan 38 25 57 14 31 +3 5.75 +5.24 * * Green Ridge Pettis 34 22 48 11 29 +4 3.51 +3.04 * * Lamar Barton 39 24 56 10 32 +2 3.96 +3.43 * * Cook Station Crawford 42 28 54 15 35 +4 3.13 +2.38 * * Delta Cape Girardeau 46 35 57 26 40 +8 2.56 +1.69 * * Cardwell Dunklin 51 39 65 29 45 +12 2.67 +1.57 * * Clarkton Dunklin 50 36 65 29 43 +10 3.16 +2.08 * * Glennonville Dunklin 50 36 64 28 43 +10 3.59 +2.53 * * Charleston Mississippi 50 38 65 30 43 +11 3.63 +2.56 * * Portageville- Delta Center Pemiscot 51 38 66 30 44 +10 3.43 +2.34 * * Portageville- Lee Farm Pemiscot 51 39 66 30 45 +11 2.64 +1.55 * * Steele Pemiscot 52 40 66 30 46 +12 2.55 +1.51 * * * Complete data not available for report ^Growing degree days are calculated by subtracting a 50 degree (Fahrenheit) base temperature from the average daily temperature. Thus, if the average temperature for the day is 75 degrees, then 25 growing degree days will have been accumulated. Pat Guinan, Commercial Agriculture Program (573) 882-5908